1. Field of the Invention
The invention relates to a ship drive with two counterturning propellers, the rear propeller of which is fastened to an inner shaft and the front propeller of which is fastened to the head end of a hollow shaft, which runs concentrically with the inner shaft, the shafts being connected to a gearing, and the input shaft of which being connected to a propulsion unit.
2. Description of the Prior Art
Propellers, rotating in opposite directions, have previously been used predominantly for smaller, conventional ship propulsion systems, for outboard motors and for sporting boats. In this respect, reference is made to the German patent 9,000,354. So-called CRP (contra rotating propeller) propulsion systems are also used for larger merchant vessels. For example, from the German publication 39 39 187 C2, a ship's propulsion is known, for which, with one main power plant, two propellers or screws are disposed on the same axle and a coaxial double axle of an inner and an outer shaft is gear reduced with respect to the rpm. As gearing, a planetary gear is used for the ship's propulsion known from this publication. On the one hand, the large number of individual elements is a disadvantage of this arrangement. In particular, the high number of planet wheels, which can reach the number 7 for the transmission of all input torque of 10,000 kNM, is disadvantageous here. Moreover, emergency operations are not possible in the event of difficulties with the propulsion unit or defects in components of the planetary gear. A failure of one of the elements of planetary gear usually leads inevitably to the destruction of the whole system, as a result of which the ship becomes disabled.
Furthermore, the requirement of a high propeller efficiency demands a precisely defined rpm ratio of the screws operating in opposite directions. Advantageous efficiency values are obtained when the inner propeller rotates 10% to 50% more rapidly than the outer one. By these means, however, transmission ratios arise for the usually used planetary gears which, due to the finite diameter of the planet wheel, can be realized technically in one step only at about 1:1.3. This problem then forces the designer to use stepped planets, which increase the number of gear parts even further in a disadvantageous manner.